Maybe be honest as far as the activity itself, but try to downplay the extent of the activity. Admit to the fact that you went to an MP, but it was a fluke, you didn't know what you were doing, you don't know why, you just needed some relaxation but made the wrong choice, you feel horrible about it, it will never happen again, you thought it was just an RMT place but in fact more happened than you expected, beg for mercy from this one silly episode, you were so relaxed that you were just about asleep when you realized what was happening and by then it was too late, etc., etc.
By the way, this fear is exactly why I'm perfectly happy with just a standard rub and tug. I find it just so enjoyable being 100% naked in front of a woman who is paying absolute attention to my body and my needs. The fact that the MPA is fully or partially clothed, to me at least as a married man, helps to reduce the guilt, and in a weird way, even enhances the excitement of what is happening. In the US, I know that the Centers for Disease Control (CDC) states that touching and massage is perfectly safe. Beyond that, no thanks - I just don't think it is worth the health risk, no matter how slight, especially for a married man.